FAW's Impact on SSIs

“How can forced-air warming be unsafe when research shows that it reduces infections?”

This is a common question, and the best explanation was given by Yale scientist Dr. Marc Abreu in the October 2010 Anesthesiology News Review. In the article, titled New Concepts in Perioperative Temperature Management: Monitoring and Management, Dr. Abreu wrote that, “…any potential airborne contamination caused by FAW waste heat must have a negligible effect on soft tissue SSIs. However, patients undergoing surgery involving implanted foreign materials, especially for orthopedics, may be at higher risk for infection from airborne contamination.” (emphasis added)

The risk of infection with FAW, at least as established so far, is specific to ultra-clean surgeries involving implanted foreign materials: orthopedics, neuro, and cardiac. There is a fundamental difference in causation between soft-tissue infections and surgical infections involving implanted foreign materials—and that is the point commonly missed.



In a seminal article published in 1996, Kurz showed that FAW decreases the SSI rate by a factor of 3x in soft tissue, colon surgery.(1) This narrow finding has been generalized to the belief that all patients treated with FAW are at a lower risk for infections for all surgeries.

The error is in the generalization of Kurz’s very specific data: research specific to soft tissue wound infections in colon surgery has been illogically generalized to apply to all wound infections. The Kurz study is excellent work, however the causative mechanisms for soft tissue infections are fundamentally different than for deep joint infections.

Soft Tissue Infections:
It takes more than 10,000 bacteria contaminating a wound to cause a soft tissue infection, and those bacteria usually get into the wound from the adjacent skin or from cut bowel. The Kurz study certainly validates that normothermic patients have fewer soft tissue infections, and it probably does not matter if the warming technique (FAW) also contaminates the air above the wound: it’s difficult to get 10,000 bacteria into an open wound by the airborne route.

Deep Joint Infections:
In contrast, it takes only a single bacterium to infect a new prosthetic joint (and probably any other surgery involving implanted foreign materials), and that bacterium usually gets into the wound by the airborne route.(2,3) The Kurz study does not address this situation. Once the bacterium lands on the implanted foreign material, it encapsulates itself in biofilm, which protects it from both antibiotics and antibodies. In contrast, the bacteria in soft tissue wounds cannot form effective biofilms.

Until recently, deep orthopedic infection rates during FAW had never been formally studied. Considering the fundamentally different mechanism of causation, the safety of FAW during implant surgery cannot be assumed from a soft tissue infection study such as Kurz’s.

A new study involving 1437 patients over 2.5 years, has finally addressed this issue. The conclusion:

“[Bair Hugger®] Patient warming ventilation disruption was associated with a significant increase in deep joint infections, as demonstrated by an elevated infection odds-ratio (3.8, p=0.028) for the forced air versus conductive fabric patient groups (n=1437 cases, 2.5-year period).”(4) (emphasis added)

To put it more simply: Patients warmed with forced-air suffered 3.8 times more deep joint infections than those warmed after forced-air was discontinued.

In surgeries that involve implanted materials, especially orthopedic implants such as knees and hips, the new research shows that FAW increases deep joint infections, as compared to other warming modalities that do not blow hot, contaminated air.

The long-held assumption that FAW decreases SSIs is simply not valid for implant surgery.

References:
1. Kurz A, et al. Perioperative normothermia to reduce the incidence of surgical-wound infection and shorten hospitalization. NEJM. 1996; 334:1209-1215.
2. Whyte W. The role of clothing and drapes in the operating room. J. Hosp. Infect. 1988 May; 11 Suppl C:2-17.
3. Mangram AJ, Horan TC, Pearson ML, Silver LC, Jarvis WR. Guideline for Prevention of Surgical Site Infection, 1999. Centers for Disease Control and Prevention (CDC) Hospital Infection Control Practices Advisory Committee. Am J Infect Control. 1999 Apr;27(2):97-132; quiz 133-134; discussion 96.
4. Forced Air Warming and Ultra-Clean Ventilation Do Not Mix: An Investigation of Theatre Ventilation, Patient Warming and Joint Replacement Infection in Orthopedics. Study accepted for publication in a major orthopedic surgery journal, November 2011.